Added `GetSize()` const method for FixedStringType #693

Job	Run time
build (ubuntu-20.04, clang-6, ssl_ON, dependencies_BUILT_IN)	3m 45s
build (ubuntu-20.04, clang-6, ssl_OFF, dependencies_BUILT_IN)	3m 46s
build (ubuntu-20.04, clang-10-libc++, ssl_ON, dependencies_BUILT_IN)	4m 25s
build (ubuntu-20.04, clang-10-libc++, ssl_OFF, dependencies_BUILT_IN)	3m 56s
build (ubuntu-20.04, gcc-7, ssl_ON, dependencies_BUILT_IN)	3m 35s
build (ubuntu-20.04, gcc-7, ssl_OFF, dependencies_BUILT_IN)	3m 21s
build (ubuntu-20.04, gcc-8, ssl_ON, dependencies_BUILT_IN)	3m 32s
build (ubuntu-20.04, gcc-8, ssl_OFF, dependencies_BUILT_IN)	3m 32s
build (ubuntu-20.04, gcc-9, ssl_ON, dependencies_BUILT_IN)	4m 17s
build (ubuntu-20.04, gcc-9, ssl_OFF, dependencies_BUILT_IN)	4m 13s
build (dependencies_SYSTEM, compiler_SYSTEM, ubuntu-22.04, gcc g++, gcc, g++, libabsl-dev liblz4-...	4m 7s
	42m 29s

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