fix!: spec version field to follow semver

[rdimitrov]

The following PR updates the initial spec_version values for all metadata roles so they comply with the specification by following the SemVer format.

Note that if there's an implementation that depends on a non-spec compliant versioning it may be affected by this change, so thus why it is marked as breaking.

Related #206

Release Notes:

Types of changes:

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to not work as expected). Please ensure that your PR title is a Conventional Commit breaking change (with a !, as in feat!: change foo).

Description of the changes being introduced by the pull request:

Please verify and check that the pull request fulfills the following requirements:

• [x] Tests have been added for the bug fix or new feature
• [ ] Docs have been added for the bug fix or new feature

[rdimitrov]

Would this break new clients on old metadata? Could/should we make it tolerant of the exact string "1.0"?

From what I've seen I haven't found a place where go-tuf actually takes into account the format of the spec_version value, so thus why I think it shouldn't break any new or existing go-tuf clients.

Nevertheless, I've marked it as breaking due to the fact that there might be some other client implementation (go or something else) that actually takes into account this format.

[rdimitrov]

Closing this one since the code base change and is no longer relevant