feat(firestore): add support for remaining string expressions

[dlarocque]

Adds support for string expressions ltrim, rtrim, stringRepeat stringReplaceOne stringReplaceAll, stringIndexOf.

Ported from: googleapis/java-firestore#2326

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[github-actions]

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[dlarocque]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces several string manipulation functions to the Firestore pipeline API, including ltrim, rtrim, stringRepeat, stringReplaceAll, stringReplaceOne, and stringIndexOf. These functions are added to the Expression class, updated in the public API definition, and supported by new unit tests. The review feedback identifies a duplicate 'substring' key in the FunctionRegistry map that should be removed to avoid redundancy and potential confusion.

[dlarocque]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces several new string-related pipeline expressions to Firestore, including ltrim, rtrim, stringIndexOf, stringRepeat, stringReplaceOne, and stringReplaceAll. The changes encompass API definition updates, DSL implementations in the Expression class, and comprehensive instrumentation tests for each new function. One minor issue was identified in the CHANGELOG.md where a pull request number in the link text was inconsistent with the actual URL.