Add a stubbed getFeeEstimate gRPC API

[steven-sheehy]

Description:

Add a stubbed getFeeEstimate gRPC API

Related issue(s):

Fixes #12099

Notes for reviewer:

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

[lfdt-bot]

🎉 Snyk checks have passed. No issues have been found so far.

✅ security/snyk check is complete. No issues have been found. (View Details)

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
❌ -31.44% (target: -1.00%)	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (6062fc4)	36859	33568	91.07%
Head commit (484f066)	60853 (+23994)	36289 (+2721)	59.63% (-31.44%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#12115)	18	18	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[xin-hedera]

LGTM