Deprecate `fromString` method in `PrivateKey`

[rokn]

[thenswan]

For reference:

• relevant JS SDK issue;
• relevant JS SDK PR.

I reproduced same scenario in Java SDK of getting an error only when using an ECDSA HEX encoded private key. But would like to mention a couple of points to discuss them before opening relevant PR:

• PrivateKey.fromString() has a lot of usages in examples and tests -- should they be updated along with deprecation? (Similar situation in Go SDK)
• there is also a PrivateKey.fromBytes() method, which may give an error on some valid key -- should we consider its deprecation in SDKs as well?

Although deprecation of PrivateKey.fromString() is straightforward, would like to call this out to refresh our memory on a PrivateKey class and review consistency across all of the SDKs in it.

cc @agadzhalov

[thenswan]

Try to introduce a fix here.