Add Apache license header to spans.rs

[alamb]

• part of Release sqlparser-rs version 0.53.0 / sqlparser_derive 0.3.0 #1517

While trying to make a release candidate, the license check failed

Running rat license checker on /Users/andrewlamb/Software/sqlparser-rs/dev/dist/apache-datafusion-sqlparser-rs-0.53.0-rc1/apache-datafusion-sqlparser-rs-0.53.0.tar.gz
NOT APPROVED: src/ast/spans.rs (apache-datafusion-sqlparser-rs-0.53.0/src/ast/spans.rs): false

This PR adds the license to spans.rs (a newly added file)

[alamb]

I plan to cherry-pick this commit to a release branch so it doesn't block the release

[iffyio]

LGTM!

[alamb]

Thanks @iffyio