Refactor out more usage of servenv for mysql version

[dbussink]

This refactors out usage of servenv inside the evalengine. This helps evalengine be more of a standalone dependency, so it's easier to use and pulls in way fewer dependencies when used as a library

Transitively this also helps schemadiff which ends up using evalengine for view semantic analysis and heavily reduces schemadiff dependencies as well (together with the refactor also done in #14930. This is similar in nature as well as #14822, #14781 & #14732

Related Issue(s)

Part of #14717 and when used for https://github.com/planetscale/schemadiff greatly reduces dependencies (and binary size) there.

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: 52 lines in your changes are missing coverage. Please review.

Comparison is base (eddb39e) 47.29% compared to head (58731a2) 47.31%.
Report is 3 commits behind head on main.

Files	Patch %	Lines
...gate/planbuilder/operators/info_schema_planning.go	0.00%	9 Missing ⚠️
...o/vt/vttablet/tabletmanager/vreplication/engine.go	28.57%	5 Missing ⚠️
go/vt/vtgate/planbuilder/operators/route.go	0.00%	4 Missing ⚠️
go/vt/vtctl/grpcvtctldserver/server.go	40.00%	3 Missing ⚠️
go/vt/vttablet/onlineddl/executor.go	0.00%	3 Missing ⚠️
go/vt/vttablet/tabletserver/schema/engine.go	25.00%	3 Missing ⚠️
go/vt/vttablet/tabletserver/tabletenv/env.go	0.00%	3 Missing ⚠️
go/vt/vtgate/evalengine/expr_env.go	80.00%	2 Missing ⚠️
go/vt/vtgate/planbuilder/operators/insert.go	0.00%	2 Missing ⚠️
go/vt/vtgate/vtgate.go	0.00%	2 Missing ⚠️
... and 13 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14938      +/-   ##
==========================================
+ Coverage   47.29%   47.31%   +0.01%     
==========================================
  Files        1137     1137              
  Lines      238684   238718      +34     
==========================================
+ Hits       112895   112940      +45     
- Misses     117168   117169       +1     
+ Partials     8621     8609      -12     

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[dbussink]

@codecov
codecov/project — 47.29% (-0.01%) compared to eddb39e

@mattlord Is there maybe some threshold that needs to be configured? This kinda reads like a rounding error / issue of why it failed?