der: Generic custom-class long > 30 tags PRIVATE, CONTEXT-SPECIFIC, APPLICATION

[dishmaker]

• Continuation of:
  PR support private tags and tag numbers >30 that are stored in long form #1416
  Issue der: Encoding higher tag number #1381
  Issue der: implementing other tag classes than Context-Specific #877

In this PR:

• Allow the use of tag numbers >30. In the der representation they are stored in the long form as multiple bytes.

• Allow APPLCATION and PRIVATE tags to be annotated like context specific ones in the derive macros

• Merge application, private, context-specific tags into one generic codebase CustomClass.

• Split EXPLICIT and IMPLICIT encoding into CustomClassExplicit/Implicit

• Add generic TAG: u16 to eg. ContextSpecificExplicit wrapper.

• Fix the need of implementing both Decode and DecodeValue on CONTEXT-SPECIFIC tags.

  • EXPLICIT tags only need to implement Decode<'a>.
  • IMPLICIT tags only need to implement DecodeValue<'a> and Tagged.
  • todo: maybe drop the requirement of Tagged too - it is needed for checking constructed bit.

• Add read_nested for IMPLICIT decoder (I had to fix x509-cert then tests, negative ones)

  In implicit Decode there was no read_nested before:

  formats/der/src/asn1/context_specific.rs

  Lines 66 to 67 in c501837

  	let header = Header::decode(reader)?;
  	let value = T::decode_value(reader, header)?;

Main breaking feature is here:

formats/der/src/asn1/context_specific.rs

Lines 9 to 23 in 756ef5f

	/// Context-specific class, EXPLICIT
	pub type ContextSpecificExplicit<const TAG: u16, T> =
	CustomClassExplicit<TAG, T, CLASS_CONTEXT_SPECIFIC>;
	/// Context-specific class, IMPLICIT
	pub type ContextSpecificImplicit<const TAG: u16, T> =
	CustomClassImplicit<TAG, T, CLASS_CONTEXT_SPECIFIC>;
	/// Context-specific class, reference, EXPLICIT
	pub type ContextSpecificExplicitRef<'a, const TAG: u16, T> =
	CustomClassExplicitRef<'a, TAG, T, CLASS_CONTEXT_SPECIFIC>;
	/// Context-specific class, reference, IMPLICIT
	pub type ContextSpecificImplicitRef<'a, const TAG: u16, T> =
	CustomClassImplicitRef<'a, TAG, T, CLASS_CONTEXT_SPECIFIC>;

[dishmaker]

Sorry for not providing documentation on ContextSpecificExplicit and AnyCustomClassExplicit etc...
I'll try maybe next week.

[brandonweeks]

I've tested this PR with a prototype for parsing the Android key attestation X.509 extension that exclusively has tags >31. No issues discovered.

https://developer.android.com/privacy-and-security/security-key-attestation#key_attestation_ext_schema