feat: implementing Startable and propagating stop()

[curtis-h]

Description:

Issue: #225

Agent.stop() now chains into Pluto.stop() which chains to Store.stop().

• Created Startable namespace to define and share implementation.
• added optional stop fn to Pluto interface,
  • avoided implementing Startable as it would be break the start() return type
• Pollux implements Startable
• Pluto
  • added Startable.State
  • improved start() fn
  • added stop() fn
• Pluto/rxdb/Store added stop() fn
• Agents
  • replaced duplicated AgentState with Startable.State
  • handled start and stop
• added functional tests

Checklist:

• My PR follows the contribution guidelines of this project
• My PR is free of third-party dependencies that don't comply with the Allowlist
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked the PR title to follow the conventional commit specification

[github-actions]

Lines	Statements	Branches	Functions
	76.9% (3574/4647)	66.88% (1555/2325)	80.79% (875/1083)

[coveralls]

Pull Request Test Coverage Report for Build 11609047595

Details

• 68 of 73 (93.15%) changed or added relevant lines in 7 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.5%) to 73.379%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/edge-agent/didcomm/Agent.ts	20	25	80.0%

Files with Coverage Reduction	New Missed Lines	%
src/domain/utils/DER.ts	3	35.14%

Totals
Change from base Build 11556271983:	0.5%
Covered Lines:	3470
Relevant Lines:	4523

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💛 - Coveralls