[VAULT-5003] Use net/http client in Sys().RaftSnapshotRestore

[VinnyHC]

What does this PR do?

Modifies (*client).Sys().RaftSnapshotRestore function to use a net/http client to prevent attempting to read the entire body of a request (which is the case in our (*client).RawRequestWithContext() )
The go-retryablehttp package makes a copy of the request body on construction of a retry-able request to be able to re-play the request. When large snapshots are streamed to the server, this is problematic for the API client’s Sys().RaftSnapshotRestore function.

Any useful background information?

This likely hasn't been identified as a problem in our CLI tool because we use files that go-retryablehttp can handle without reading the full value into memory.
Our (*client).Sys().RaftSnapshot method currently implements a net/http client.

How has this been tested?

Tests have been updated to use helper functions.
Analyzed a memory profile of our current code when writing a "large" snapshot, which copies the entire request body, where the new code does not. Large writes have not been included in the updated tests as to not gobble up unnecessary resources.
"large" data object:

	data := map[string]interface{}{"target": strings.Repeat("a", 1000000)}
	_, err := leaderClient.Logical().Write("secret/big", data)

[averche]

One optional comment, LGTM otherwise 👍

[ncabatoff]

Nice work!