Vreplication: control streamer snapshot method via rule/filter directives

[shlomi-noach]

Description

Implements a solution to #16420:

• Adding StreamerSnapshotMethod and associated field in proto/Rule
• This can be undefined/track/lock, indicating how we want the streamer to create a snapshot.
• When defined, the hint is sent as query comments, much like ukForce or ukColumns.
• The hint is interpreted by the streamer and snapshot is taken appropriately.

Right now this is just offered as a potential functionality, and the actual behavior is unchanged. In the future, we may choose to actually make educated choices to pick the preferred snapshot method.

This change is internal facing and not to be documented yet.

Related Issue(s)

Fixes #16420

Checklist

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Deployment Notes

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[codecov]

Codecov Report

Attention: Patch coverage is 90.24390% with 4 lines in your changes missing coverage. Please review.

Project coverage is 68.67%. Comparing base (f73f624) to head (4d8919e).
Report is 1 commits behind head on main.

Files	Patch %	Lines
...t/vttablet/tabletserver/vstreamer/snapshot_conn.go	69.23%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16421      +/-   ##
==========================================
- Coverage   68.69%   68.67%   -0.03%     
==========================================
  Files        1548     1548              
  Lines      199090   199097       +7     
==========================================
- Hits       136766   136721      -45     
- Misses      62324    62376      +52     

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[shlomi-noach]

Closing for now in favor of #16424. We can revisit in the future.