entc/gen: use join for loading m2m relationship

[a8m]

Rewrite the way Ent eager-loads M2M edges, by replacing two queries with a single JOIN query. The main reasons are:

1. This change is necessary for the nested-pagination implementation in entgql in order to use SQL WINDOW functions and limit the number of edges returned for each entity.
2. In the average case, Ent users limit the number of records they query from the database using entgql/entproto pagination or use limit+offset, and the previous implementation used to scan all edges even though it was not necessary (e.g. With<E> configured with Limit, Order or Where).
3. If we want to keep the previous implementation and avoid scanning all edges, we should add JOIN to the destination table which makes it irrelevant.
4. In case there are many edges, the previous implementation is reached the database parameters limit (e.g. 65535 in PostgreSQL).

Test plan

Added 10K users, and connected each one of them to a unique group with attribute active=false. Then, created another 10 groups (with active=true) and connected them to all users.

The results are as follows:

TestMySQL/8 (branch:withm2m)
users->groups(active:false)	104.482914ms
users->groups			394.802676ms
groups(active:false)->users	105.381099ms
groups->users			104.649884ms

TestMySQL/8 (branch:master)
users->groups(active:false)	176.937021ms
users->groups			180.452625ms
groups(active:false)->users	111.331736ms
groups->users			165.322574ms

When querying users->groups, the new JOIN of users.id + groups columns generate 99990 duplicate rows, unlike the previous solution, that used 2 queries. One for getting all edges (without filtering) from join-table, and another one for querying the destination table (the one requested on With).
However, in cases like groups(active:false) when there are no duplicate rows at all, the numbers are much better.

Future work

If we'll feel it is necessary, we can provide users an option to configure the way relationships are queried using "runtime annotation/hints". Some rough ideas I have in mind:

client.User.Query().
    WithGroups(sql.LoadWithJoin()).
    WithFriends(sql.LoadWithSubQuery()).
    All(ctx)

[masseelch]

The tests did not pick up the bug, did they? Should we add a test for it?

[masseelch]

💯