modem: cmux: Define encoding and decoding functions for commands

[SeppoTakalo]

Instead of relying non-standard compiler behavior, define encode and decode functions for all CMUX command structures.

Drop:

struct modem_cmux_command *modem_cmux_command_wrap(const uint8_t *data);
static int modem_cmux_wrap_command(struct modem_cmux_command **command, const uint8_t *data,
				   uint16_t data_len);

And replace with:

static struct modem_cmux_command_type modem_cmux_command_type_decode(const uint8_t byte);
static uint8_t modem_cmux_command_type_encode(const struct modem_cmux_command_type type);
static struct modem_cmux_command_length modem_cmux_command_length_decode(const uint8_t byte);
static uint8_t modem_cmux_command_length_encode(const struct modem_cmux_command_length length);
static struct modem_cmux_command modem_cmux_command_decode(const uint8_t *data, size_t len);
static int modem_cmux_command_encode(uint8_t *buf, size_t len,
				     const struct modem_cmux_command *command);

as previous _wrap_ functions were relying on:

struct modem_cmux_command_type {
	uint8_t ea: 1;
	uint8_t cr: 1;
	uint8_t value: 6;
};

to be placed correctly. But as discussed on PR #97569 here this is non-standard behavior.
Works on GCC on Arm and seem to be working on GCC on X86_64 native_sim, but we should not rely on that.

[SeppoTakalo]

Fixed the compilation issues.
That was a real bug... not GCC failure.

Looks like we need to have static buffer instead of flexible array member, because we end up decoding the whole command and its included data.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
1 Accepted issue

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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