Add new metadata folder to hold ODC product definition and metadata type

[robbibt]

This PR adds a new folder called metadata to contain metadata files required for indexing DEA Intertidal into the Open Data Cube (ODC). This includes:

• An ODC Product Definition YAML describing the DEA Intertidal product and its bands/measurements
• An ODC Metadata Type YAML defining custom searchable metadata fields for DEA Intertidal

The two datasets above can be validated against an ODC Dataset Document for each tile using the eo3-validate command from eodatasets:

!eo3-validate \
metadata/ga_s2ls_intertidal_cyear_3.odc-product.yaml \
metadata/eo3_intertidal.odc-type.yaml \
data/processed/ga_s2ls_intertidal_cyear_3/0-0-1/tes/ting/2023--P1Y/ga_s2ls_intertidal_cyear_3_testing_2023--P1Y_interim.odc-metadata.yaml \
--thorough

[github-actions]

For full integration test results, refer to the Tests directory README.

[vnewey]

Looks great!