feat: always generate agreement key on startup

[edward-swirldslabs]

Description:

This PR stops reading the agreement key from disk and always generates it on startup. The migration code will delete the agreement key if found and force generation anyway. This is a code cleanup operation so that when the migration code is removed, there are no potential bugs.

Potential Bug This Avoids:

• PFX file contains an agreement key
• Migration code is deleted and PFX files remain on disk because someone put them there again.
• DAB has rotated the signing key and is using a new signing key in a PEM file.
• Agreement key for old signing key is loaded from PFX file.
• The mismatch between the old agreement key and new signing key will cause the node to fail to create mTLS connections with peers.

Fixes #16161

Notes for reviewer:
The only testing required and performed was unit testing. The special casing on one of the negative unit tests was removed because we are no longer reading a valid agreement key in that scenario.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ -0.01% (target: -1.00%)	✅ 0.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (3daf1bf)	100220	62084	61.95%
Head commit (1f76a44)	100216 (-4)	62070 (-14)	61.94% (-0.01%)

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 (#16315)	1	0	0.00%

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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