dsssframe: allow specifying spreading factor

[glv2]

Add the dsssframegen_create_set and dsssframesync_create_set functions that can
be used to specify a spreading factor different from 64.

[codecov]

Codecov Report

Merging #285 (c8fed4d) into master (2f469e9) will decrease coverage by 0.00%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master     #285      +/-   ##
==========================================
- Coverage   77.42%   77.42%   -0.01%     
==========================================
  Files         335      335              
  Lines       24752    24760       +8     
==========================================
+ Hits        19165    19171       +6     
- Misses       5587     5589       +2     

Impacted Files	Coverage Δ
src/framing/src/dsssframegen.c	86.40% <88.88%> (-0.23%)	⬇️
src/framing/src/dsssframesync.c	84.70% <90.00%> (-0.15%)	⬇️

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