Add after_key to Map

[Yesterday17]

According to RFC 8216, section 4.3.2.4:

Media Segments MAY be encrypted. The EXT-X-KEY tag specifies how to
decrypt them. It applies to every Media Segment and to every Media
Initialization Section declared by an EXT-X-MAP tag that appears
between it and the next EXT-X-KEY tag in the Playlist file with the
same KEYFORMAT attribute (or the end of the Playlist file).

Therefore, an initial section declared by EXT-X-MAP can be unencrypted if it appears before the first EXT-X-KEY tag. However, m3u8-rs currently does not store this location information.

This pull request introduces a flag called after_key to save this information.

The naming of this flag might be inappropriate and needs to be revised.