ctf: refactor metadata decoder to always have an instance

ctf: refactor metadata decoder to always have an instance

This patch changes the internal `ctf` plugin's metadata decoder API so
that you need to create a decoder instance to read properties instead of
calling ctf_metadata_decoder_packetized_file_stream_to_buf().

Now, you create a metadata decoder, you call
ctf_metadata_decoder_append_content() (similar to what was called
ctf_metadata_decoder_decode()), and then you can get the decoder's
current properties:

* ctf_metadata_decoder_get_ir_trace_class()
* ctf_metadata_decoder_borrow_ctf_trace_class()
* ctf_metadata_decoder_get_byte_order()
* ctf_metadata_decoder_get_uuid()
* ctf_metadata_decoder_get_text()

The decoder's configuration contains two new flags:

`create_trace_class`:
    Controls whether or not ctf_metadata_decoder_append_content()
    invokes the AST visitor to generate trace class objects.

    Trace classes are not needed, for example, for the `metadata-info`
    query.

`keep_plain_text`:
    Controls whether or not the decoder accumulates the metadata
    stream's plain text when you call
    ctf_metadata_decoder_append_content().

    This needs to be true to use ctf_metadata_decoder_get_text().

The goal of all this is to eventually add new metadata property getters
which do not need to create trace classes or to keep the metadata plain
text.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: If1095da91aee0f446d7f5efcec765e5abbb2b30a
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1842
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>