Add spec file for metadata.

[Nepomuceno]

First step trying to improve the specs at #275

What it does

Add the basic Spec file fore metadata.
This is inspired in the file of https://github.com/sigmf/sigmf-python/blob/main/sigmf/schema-meta.json

Assumptions

these assumprions need to be verified

• There is a minimum of one channel that it is specified in the files.
• Sample retes are always positive
• The only version current cupported by this schema it is 1.0.0
• Frequencies can be negative

What it is missing

In his pr it is still missing the implementation of the extensions but space it is given to them.

I did not have many sample files to tesst the validation of the schema if someone could help me with that would be great.

[777arc]

1. There is a minimum of one channel that it is specified in the files, correct, an empty captures array would negate the need for the recording to exist imo
2. Sample rates are always positive, correct
3. Yep it's OK if the latest release (1.0.0) is the only one supported, although we should talk about what the process is when a new release is added, e.g. we should archive the older schemas in a nice way
4. Frequencies can be negative <currently in discussion in sigmf matrix chat, will update>

[777arc]

Is it possible to make an example of having a separate schema file for an extension, e.g. it could be a 1 field extension but just showing how the $ref stuff will look, and then people who have extensions can use it as an example for making their schema file? Not sure how much work that would be.

[romanziske]

The json schema is missing the core:geolocation field.

[777arc]

closing because this will be included in the #292