btf: move Spec.Add to dedicated type

[lmb]

internal: add NewBuffer

Share a single pool of bytes.Buffer across the whole library.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

btf: add marshalOptions.Order

Allow specifying which ByteOrder to encode BTF into via a new field in
marshalOptions.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

btf: use number of types to pre-size string table

Remove the dependency on the ELF string table to pre-size the string table
builder for large Specs. Instead we use the heuristic that for vmlinux the
number of strings is roughly equal to the number of types.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

btf: refactor to Builder

Spec allows both querying Types as well as constructing new BTF blobs via
Add. This has some unfortunate consequences: we need to do extra work so
that Added types can be queried and query code needs to be aware that the
set of types can change. The query API also allows observing side-effects of
adding types which we'd rather hide, for example when working around datasec
bugs. It's also difficult to make the implementation efficient for both use
cases at once.

Split the code for constructing new BTF blobs into a separate struct.

NewHandle now takes a Builder instead of a Spec which is a breaking change.
Other API changes are to functions which have not appeared in a release yet.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

btf: unexport NewSpec

NewSpec was added to be able to create BTF from scratch using Spec.Add. This
usecase was migrated to Builder and Spec.Add doesn't exist anymore.

Unexport NewSpec so we can still use it for testing but users only
experience API breakage once.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

btf: generalise datasecWorkaround to Builder.Add

Extend the datasec validation workaround to all Datasec. This is safe to do
because there are no methods on Builder which allow going from TypeID to
Type.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

[rgo3]

Only nits, human linter goes brrr. Otherwise LGTM.