lwm2m: Add offset to post-write callback

[SeppoTakalo]

This PR contains fix for #71351 and tests to verify that, but also new fixes for issues that I saw while implementing the tests.

lwm2m: Add offset to post-write callback

When Block-Wise transfer restarts, the post-write callback should
receive some indication that the block is actually a beginning of new,
instead of part of previous transfer.

NOTE: This is API break.

Fixes #71351
Fixes #71269

test: lwm2m: Add tests for Block-Wise transfers

Block-Wise GET, PUT and SEND

lwm2m: Allow Block-Wise write without callbacks

Allow Block-Wise writes to resources that have enough buffer for
the whole content. Calculate the offset before calling the
content specific write handlers when callback is not set.

Previously only resources with post-write callback were able to
assemble the content.

test: lwm2m: Enable DTLS Connection ID

Enable connection ID because it is on by default on the
client example as well.

[SeppoTakalo]

APi changes can be quickly refactored with following Coccinelle script

@@
identifier write_cb;
identifier oid;
identifier rid;
identifier riid;
identifier data;
identifier len;
identifier last;
identifier size;
type uint16_t;
@@

- int write_cb(uint16_t oid, uint16_t rid, uint16_t riid, uint8_t *data, uint16_t len, bool last, size_t size)
+ int write_cb(uint16_t oid, uint16_t rid, uint16_t riid, uint8_t *data, uint16_t len, bool last, size_t size, size_t offset)
{
	...
}

To use it, save as ~/refactor_post_write_cb.spatch and run

spatch --sp-file ~/refactor_post_write_cb.spatch --in-place src/*.c

[rlubos]

Looks good! Would be good to add an entry in the migration guide about the API change.

[marco85mars20]

Looks great! Thanks for implementing this fix @SeppoTakalo

[SeppoTakalo]

Added a migration guide and fixed the copyright date.

[aescolar]

A few minor things

Addressed

[marco85mars20]

LGTM

[marco85mars20]

This PR also fixes #71269