Add fuzz target for msgpack tovalue

[schrewe]

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[github-actions]

File	Coverage
include/tao/json/jaxn/events
from_input.hpp	100%
from_string.hpp	100%
include/tao/json/msgpack/events
from_input.hpp	100%
from_string.hpp	100%
.code-intelligence/fuzz_targets
ubjson_validation.cpp	100%
jaxn_validation.cpp	92%
json_validation.cpp	92%
msgpack_validation.cpp	92%
include/tao/json/ubjson/events
from_input.hpp	100%
from_string.hpp	86%
external/PEGTL/include/tao/pegtl/internal
iterator.hpp	100%
sor.hpp	100%
bump_help.hpp	100%
eof.hpp	100%
seq.hpp	100%
input_pair.hpp	100%
must.hpp	100%
star.hpp	100%
opt.hpp	100%
peek_utf8.hpp	90%
marker.hpp	90%
one.hpp	83%
peek_char.hpp	83%
range.hpp	83%
action_input.hpp	80%
plus.hpp	75%
if_must.hpp	71%
bump.hpp	46%
include/tao/json/events
from_input.hpp	100%
from_string.hpp	100%
limit_nesting_depth.hpp	100%
validate_event_order.hpp	32%
external/PEGTL/include/tao/pegtl
config.hpp	100%
match.hpp	98%
change_states.hpp	92%
parse.hpp	89%
memory_input.hpp	74%
normal.hpp	63%
parse_error.hpp	45%
position.hpp	44%
demangle.hpp	38%
include/tao/json/ubjson/internal
grammar.hpp	59%
include/tao/json/internal
endian.hpp	100%
endian_gcc.hpp	92%
parse_util.hpp	72%
format.hpp	70%
number_state.hpp	35%
errors.hpp	26%
grammar.hpp	18%
action.hpp	18%
include/tao/json
utf8.hpp	78%
span.hpp	8%
include/tao/json/jaxn/internal
errors.hpp	11%
grammar.hpp	10%
include/tao/json/msgpack/internal
grammar.hpp	7%