common/: add support for changelog field.

This commits allows ./xbps-src show to show the changelog field and for
changelog to be included with the -c/--changelog option via xbps-create.

Closes: #14102 [via git-merge-pr]

Manual.md

@@ -579,6 +579,9 @@ Example:
 A list is composed of three components separated by a colon: group, symlink and target.
 i.e `alternatives="vi:/usr/bin/vi:/usr/bin/nvi ex:/usr/bin/ex:/usr/bin/nvi-ex"`.
 
+- `changelog` URL to the changelog of the package, the changelog must be in plaintext.
+i.e `changelog="https://raw.githubusercontent.com/user/package/master/CHANGELOG.md"`
+
 <a id="explain_depends"></a>
 #### About the many types of `depends` variable.