Cleanup for DecodePrivateKey() functionality

[Frauschi]

• Create a new method DecodePrivateKey_ex() that gets the key to decode as parameters
• Adapt DecodePrivateKey() and DecodeAltPrivateKey() to use this new method

This removes a lot of nearly duplicate code and simplifies maintenance.

[devin-ai-integration]

🛟 Devin Lifeguard found 2 likely issues in this PR

• declare-const-pointers snippet: Declare the parameter as const DerBuffer* key (and update any called prototypes if needed) in the signature of DecodePrivateKey_ex.
• use-proper-function-visibility snippet: Change the definitions to ‘WOLFSSL_LOCAL int wolfssl_local_DecodePrivateKey(...’ and ‘WOLFSSL_LOCAL int wolfssl_local_DecodeAltPrivateKey(...);’ (or make them static) to match the visibility rule.

@Frauschi
please take a look at the above issues which Devin flagged. Devin will not fix these issues automatically.

[wolfSSL-Bot]

Can one of the admins verify this patch?

[dgarske]

Okay to test. Welcome to the team! We'll get you officially added shortly.

[Frauschi]

I fixed the failing tests regarding the WOLFSSL_BLIND_PRIVATE_KEY option as well as the first devin issue.

Should I rename the existing DecodePrivateKey() method to wolfssl_local_DecodePrivateKey()?

[SparkiDev]

No need to change names of existing functions yet.
(Unless you are reworking all the functions)

New functions need to be properly named.

[SparkiDev]

retest this please