PKCS7: decode detached SignedData in d2i_PKCS7

[kojo1]

Description

wolfSSL_d2i_PKCS7() verified the signature while decoding, so a
detached SignedData - one whose content is not in the bundle - could
never be decoded through the OpenSSL compatibility layer. OpenSSL's
d2i_PKCS7() only decodes; the content is handed to PKCS7_verify()
afterwards.

Add wc_PKCS7_DecodeSignedData(). It parses certificates, signerInfo
and content type like wc_PKCS7_VerifySignedData() and verifies an
attached bundle as before. A detached bundle without content is only
parsed: pkcs7->detached is set and the signature is left for a later
wc_PKCS7_VerifySignedData() call with the content. The existing state
machine is reused with a parseOnly argument; no structure change.

Fixes zd#22345

Testing

Added test cases:
test_wc_PKCS7_DecodeSignedData
test_wolfSSL_PKCS7_d2i_detached

Checklist

• added tests
• updated/added doxygen
• updated appropriate READMEs
• Updated manual and documentation

[github-actions]

MemBrowse Memory Report

gcc-arm-cortex-m4

• FLASH: .text +128 B (+0.1%, 204,893 B / 262,144 B, total: 78% used)

gcc-arm-cortex-m4-openssl-compat

• FLASH: .rodata -80 B, .text +1,408 B (+0.2%, 782,236 B / 1,048,576 B, total: 75% used)

gcc-arm-cortex-m4-pkcs7

• FLASH: .text +192 B (+0.1%, 217,756 B / 262,144 B, total: 83% used)

gcc-arm-cortex-m4-rsa-only

• FLASH: .text +128 B (+0.0%, 331,936 B / 1,048,576 B, total: 32% used)

gcc-arm-cortex-m7

• FLASH: .text +128 B (+0.1%, 204,893 B / 262,144 B, total: 78% used)

linuxkm-pie

• Data: __patchable_function_entries +16 B (+0.1%, 27,000 B)

linuxkm-standard

• Data: __patchable_function_entries +16 B (+0.0%, 49,784 B)
  No memory changes detected for:
• gcc-arm-cortex-m0plus
• gcc-arm-cortex-m3
• gcc-arm-cortex-m4-baremetal
• gcc-arm-cortex-m4-crypto-only
• gcc-arm-cortex-m4-dtls13
• gcc-arm-cortex-m4-min-ecc
• gcc-arm-cortex-m4-pq
• gcc-arm-cortex-m4-sp-math
• gcc-arm-cortex-m4-tls12
• gcc-arm-cortex-m4-tls13
• gcc-arm-cortex-m7-pq
• gcc-arm-cortex-m7-tls13
• stm32-sim-stm32h753