feat: provide from_pem_wo_parameter() to rebuild sepc256k1 identity from .pem without parameters

[readygo67]

Description

"dfx identity export xxx" normally export .pem file without parameter, looks like

"-----BEGIN EC PRIVATE KEY-----
MHQCAQEEIAgy7nZEcVHkQ4Z1Kdqby8SwyAiyKDQmtbEHTIM+WNeBoAcGBSuBBAAK
oUQDQgAEgO87rJ1ozzdMvJyZQ+GABDqUxGLvgnAnTlcInV3NuhuPv4O3VGzMGzeB
N3d26cRxD99TPtm8uo2OuzKhSiq6EQ==
-----END EC PRIVATE KEY-----
"

but currently agent-rs does not have a method to rebuild identity from it, this PR provides from_pem_wo_parameter() to achieve this purpose.

How Has This Been Tested?

test_secp256k1_public_key_wo_params() is defined for test.

Checklist:

• [x] The title of this PR complies with Conventional Commits.
• [x] I have edited the CHANGELOG accordingly.
• [x] I have made corresponding changes to the documentation.

[sa-github-api]

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