Fix cargo publish ( can't find dynamic_types example at examples/dynamic_types.rs or examples/dynamic_types/main.rs)

[alamb]

Which issue does this PR close?

re #2172

Rationale for this change

When the release verification script tried to publish the 'arrow' crate, it got the following error:

+ pushd arrow
/var/folders/s3/h5hgj43j0bv83shtmz_t_w400000gn/T/arrow-20.0.0.XXXXX.dVwsseAi/apache-arrow-rs-20.0.0/arrow /var/folders/s3/h5hgj43j0bv83shtmz_t_w400000gn/T/arrow-20.0.0.XXXXX.dVwsseAi/apache-arrow-rs-20.0.0 /var/folders/s3/h5hgj43j0bv83shtmz_t_w400000gn/T/arrow-20.0.0.XXXXX.dVwsseAi
+ cargo publish --dry-run
    Updating crates.io index
   Packaging arrow v20.0.0 (/private/var/folders/s3/h5hgj43j0bv83shtmz_t_w400000gn/T/arrow-20.0.0.XXXXX.dVwsseAi/apache-arrow-rs-20.0.0/arrow)
   Verifying arrow v20.0.0 (/private/var/folders/s3/h5hgj43j0bv83shtmz_t_w400000gn/T/arrow-20.0.0.XXXXX.dVwsseAi/apache-arrow-rs-20.0.0/arrow)
error: failed to verify package tarball

Caused by:
  failed to parse manifest at `/private/var/folders/s3/h5hgj43j0bv83shtmz_t_w400000gn/T/arrow-20.0.0.XXXXX.dVwsseAi/apache-arrow-rs-20.0.0/target/package/arrow-20.0.0/Cargo.toml`

Caused by:
  can't find `dynamic_types` example at `examples/dynamic_types.rs` or `examples/dynamic_types/main.rs`. Please specify example.path if you want to use a non-default path.
+ cleanup
+ '[' no = yes ']'
+ echo 'Failed to verify release candidate. See /var/folders/s3/h5hgj43j0bv83shtmz_t_w400000gn/T/arrow-20.0.0.XXXXX.dVwsseAi for details.'
Failed to verify release candidate. See /var/folders/s3/h5hgj43j0bv83shtmz_t_w400000gn/T/arrow-20.0.0.XXXXX.dVwsseAi for details.

I can reproduce it locally too

alamb@MacBook-Pro-8 arrow % cargo publish --dry-run
cargo publish --dry-run
    Updating crates.io index
   Packaging arrow v20.0.0 (/Users/alamb/Software/arrow-rs/arrow)
   Verifying arrow v20.0.0 (/Users/alamb/Software/arrow-rs/arrow)
error: failed to verify package tarball

Caused by:
  failed to parse manifest at `/Users/alamb/Software/arrow-rs/target/package/arrow-20.0.0/Cargo.toml`

Caused by:
  can't find `dynamic_types` example at `examples/dynamic_types.rs` or `examples/dynamic_types/main.rs`. Please specify example.path if you want to use a non-default path.

I don't really know what Cargo is complaining about:

$ ls examples/dynamic_types.rs
examples/dynamic_types.rs

🤷

Looks like the change came in from @tustvold 's change on #2299

What changes are included in this PR?

Add a path attribute as suggested by Cargo

Are there any user-facing changes?

no

[ursabot]

Benchmark runs are scheduled for baseline = 83fab37 and contender = 30c94db. 30c94db is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java

[tustvold]

I have no idea about this either... Perhaps @carols10cents knows what is going on here...