extmod/modtls_mbedtls: Fix key_len passed to mbedtls_pk_parse_key.

[peterzuger]

See issue #14371:

mbedtls_pk_parse_key() expects key_len to include the NULL terminator for PEM data but not for DER encoded data.

Since all PEM data starts with "-----BEGIN" this is used to check if the data is PEM.

This can be done for both v2 and v3 of mbedtls since the fundamental behaviour/expectation did not change.

What changed is that in v3 the PKCS#8 DER parser now checks that the passed key buffer was fully utilized and no bytes are remaining (all other DER formats still do not check this).

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 98.39%. Comparing base (e60e807) to head (bb981c8).

Files	Patch %	Lines
extmod/modtls_mbedtls.c	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #14385      +/-   ##
==========================================
- Coverage   98.39%   98.39%   -0.01%     
==========================================
  Files         161      161              
  Lines       21204    21206       +2     
==========================================
+ Hits        20864    20865       +1     
- Misses        340      341       +1     

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[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:   +56 +0.007% standard
      stm32:    +0 +0.000% PYBV10
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS