[rush-bridge-cache] Support read or write of cache

[dmichon-msft]

Summary

Updates @rushstack/rush-bridge-cache-plugin to use a choice parameter instead of a flag parameter, which options of "read" or "write.

• "write" matches the prior behavior of the plugin, i.e. writes build cache entries from all existing outputs of selected operations
• "read" is a best-effort replay-from-build-cache for all selected operations. Failures will display warnings but the exit code will still be 0.

Details

The existing options schema was malformed, so this technically should not be a breaking change.

Also exposes the configured Rush parallelism on the phased operation context object, so that it can be used by plugins.

How it was tested

Locally wired it into the repository and ran various permutations of rush build --bridge-cache-action=read and rush build --bridge-cache-action=write for different operation scopes.

Impacted documentation

The plugin's documentation.