feat: Embeddable APIs as "embeddable pipeline"

[tmathern]

This PR introduces wrappers for the embeddable APIs which rely somewhat on state machine to attempt simplifying the use of the embeddable APIs.

The state of the pipeline advances based on the functions called, so it is easy to "remember" (or find out) where in the embedding process we are.

Usage example for a BMFF file:

// Create the "right" pipeline type, based on the existing builder and hash type that will be used
// (Relies on the new Builder API that tells which hash type will be used
// pipeline type here is generics, as the different pipelines inherit from a base class
auto pipeline = c2pa::EmbeddablePipeline::create(make_builder(), "video/mp4");
auto source_asset = c2pa_test::get_fixture_path("video1.mp4");

// EXPECT_EQ(pipeline->hash_type(), c2pa::HashType::Bmff);
// EXPECT_EQ(pipeline->current_state(), PipelineState::init);

pipeline->create_placeholder();
// EXPECT_EQ(pipeline->current_state(), PipelineState::placeholder_created);

// hash
std::ifstream asset_stream(source_asset, std::ios::binary);
pipeline->hash_from_stream(asset_stream);
 asset_stream.close();
// EXPECT_EQ(pipeline->current_state(), PipelineState::hashed);

// Return signed bytes
auto& manifest = pipeline->sign();

Or a JPEG with data hash:

auto pipeline = c2pa::EmbeddablePipeline::create(make_builder(), "image/jpeg");
auto source_asset = c2pa_test::get_fixture_path("A.jpg");

// EXPECT_EQ(pipeline->hash_type(), c2pa::HashType::Data);
// EXPECT_EQ(pipeline->current_state(), PipelineState::init);

auto& placeholder = pipeline->create_placeholder();
// EXPECT_EQ(pipeline->current_state(), PipelineState::placeholder_created);
size_t placeholder_size = placeholder.size();

uint64_t embed_offset = 20;
pipeline->set_exclusions({{embed_offset, placeholder_size}});
// EXPECT_EQ(pipeline->current_state(), PipelineState::exclusions_configured);

std::ifstream asset_stream(source_asset, std::ios::binary);
pipeline->hash_from_stream(asset_stream);
asset_stream.close();
// EXPECT_EQ(pipeline->current_state(), PipelineState::hashed);

auto& manifest = pipeline->sign();
// EXPECT_EQ(pipeline->current_state(), PipelineState::pipeline_signed);