grpc-xDS: reserve vector size

[adisuissa]

Commit Message: grpc-xDS: reserve vector size
Additional Description:
Modifying the watch-map to reserve a vector size prior to adding elements to it.

Risk Level: low - c++ internal only modification
Testing: N/A
Docs Changes: N/A
Release Notes: N/A - no user facing changes
Platform Specific Features: N/A

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Caused by: #36230 was opened by adisuissa.

see: more, trace.