chore: sort exceptions properly (manual synth)

[tseaver]

I'm hoping that a manual synth will get the hashes in synth.metadata updated all at once, so that future "self" and "synthtoolls" autosynth runs can use the current latest version of googleapis.

• Restore path helper methods to generated clients.
• Enforce that 'request' argument to generated client methods is exclusive
  to flattened arguments.
• Surface 'transport' property for generated clients.

Closes #251
Closes #252

[tseaver]

@crwilcox, @czahedi looks like @busunkim96 can't approve-for-merge here, either (requiresCodeOwnerReviews: true only allows members of @googleapis/firestore-dpe, looks like).

[crwilcox]

@tseaver this description seems inaccurate. Since you have reviewed fully can you describe the changes this is making (it doesn't seem to be about exception order)

[tseaver]

@crwilcox The main point of this PR is to do a full manual synth, trying to get around the fact that the three autosynth PRs can revert to older hashes for (especially, right now) googleapis.

The symptom of that reversion is that the self and synthtools builds are generating using gapic-generator-python 0.33.2, which has the bug of scrambling exception order (see #251, #253).

The rest of the changes (adding path helper methods, transport, etc.) are detailed in the commit description.