audioencoder: Add finish vfunc for subclass to be able to drain pending data

Depending on subclass implementation, there might be pending/queued encoded
data. Similar to GstVideoEncoder::finish(), GstAudioEncoder needs to
provide such a way for subclass to drain pending data on EOS.
Status Job ID Name Coverage
  Preparation
passed #2765133
gst indent

00:00:28

manual #2765132
manual
manifest
 
  Build
created #2765142
gstreamer
build cerbero cross-android universal
created #2765146
gst-ios-13.2
build cerbero cross-ios universal
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gstreamer
build cerbero cross win32
created #2765144
gstreamer
build cerbero cross win64
created #2765141
gstreamer
build cerbero fedora x86_64
created #2765145
gst-macos-10.15
build cerbero macos x86_64
created #2765137
build clang fedora x86_64
created #2765140
1809 windows docker allowed to fail manual
build msys2
created #2765134
build nodebug fedora x86_64
created #2765135
build static fedora x86_64
created #2765136
build static nodebug fedora x86_64
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1809 windows docker
build vs2017 amd64
created #2765139
1809 windows docker
build vs2017 x86
 
  Test
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gstreamer
check fedora
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integration testsuites fedora 1/4
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integration testsuites fedora 2/4
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gstreamer
integration testsuites fedora 3/4
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gstreamer
integration testsuites fedora 4/4
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gstreamer
valgrind bad
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gstreamer
valgrind base
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gstreamer
valgrind ges
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valgrind good
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  Integrate
created #2765157
cross-android universal examples
created #2765159
gst-ios-13.2
cross-ios universal examples
created #2765158
documentation