Store key as <pubkey>.key

[fasmat]

Related to spacemeshos/go-spacemesh#5643

Instead of <identity>.key store the key with the public key as the file name (abbreviated).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.3%. Comparing base (6c818a7) to head (c5e376a).

Additional details and impacted files

@@           Coverage Diff           @@
##           develop    #275   +/-   ##
=======================================
  Coverage     74.3%   74.3%           
=======================================
  Files           29      29           
  Lines         1543    1543           
=======================================
  Hits          1147    1147           
  Misses         258     258           
  Partials       138     138           

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[poszu]

Could you also update the readme:

post/cmd/postcli/README.md

Lines 101 to 107 in fdd5647

	* If `-id` isn't provided a new identity will be auto-generated. Its private key will be stored in `identity.key` in
	`-datadir` next to the PoST data. This file then **must** to be copied/moved to the `data/identities` directory of
	the node managing this identity.
	* **NOTE:** The generated PoST data is ONLY valid for this identity!
	* If a public key is provided with the `-id` flag, the `identities.key` file will be NOT created. Make sure that the
	key file that belongs to the identity provided to `postcli` is available in the `data/identities` directory for a
	PoST service using the generated data to be able to connect to the node.

[fasmat]

Closing as having "random" key names might be confusing and the change makes the code more convoluted than necessary.