add charset modifier to secret generation (#521)

since we need special chars in passwords for a recipe we are working on, i have added the option to specify a charset in the same way as the length can be setted.
i did not change anything in the behaviour, so if length is not specified, the charset gets ignored whether it is there or not.

you can specify the following:
`charset=default` - Results in passgen.AlphabetDefault being used
`charset=special` - Results in passgen.AlphabetSpecial being used
`charset=safespecial` - Results in `!@#%^&*_-+=` being used (so it is AlphabetSpecial without the dollar sign)
`charset=default,special` or `charset=special,default` - Results in passgen.AlphabetDefault + passgen.AlphabetSpecial being used
`charset=default,safespecial` or `charset=safespecial,default` - Results in passgen.AlphabetDefault + `!@#%^&*_-+=` being used ((so it is AlphabetSpecial without the dollar sign)

PR for the docs: toolshed/docs.coopcloud.tech#271

Co-authored-by: p4u1 <p4u1@noreply.git.coopcloud.tech>
Reviewed-on: toolshed/abra#521
Reviewed-by: p4u1 <p4u1@noreply.git.coopcloud.tech>
Co-authored-by: Apfelwurm <Alexander@volzit.de>
Co-committed-by: Apfelwurm <Alexander@volzit.de>

pkg/secret/testdir/.env.sample

@@ -1,3 +1,6 @@
 SECRET_TEST_PASS_ONE_VERSION=v2
 SECRET_TEST_PASS_TWO_VERSION=v1 # length=10
 SECRET_TEST_PASS_THREE_VERSION=v2
+SECRET_TEST_PASS_FOUR_VERSION=v1 # length=12 charset=default,safespecial
+SECRET_TEST_PASS_FIVE_VERSION=v1 # length=12 charset=default,special
+SECRET_TEST_PASS_SIX_VERSION=v1 # charset=default,special

pkg/secret/testdir/compose.yaml

@@ -8,6 +8,9 @@ services:
       - test_pass_one
       - test_pass_two
       - test_pass_three
+      - test_pass_four
+      - test_pass_five
+      - test_pass_six
 
 secrets:
   test_pass_one:
@@ -19,3 +22,12 @@ secrets:
   test_pass_three:
     external: true
     name: ${STACK_NAME}_pass_three_${SECRET_TEST_PASS_THREE_VERSION} # secretId and name don't match
+  test_pass_four:
+    external: true
+    name: ${STACK_NAME}_test_pass_four_${SECRET_TEST_PASS_FOUR_VERSION}
+  test_pass_five:
+    external: true
+    name: ${STACK_NAME}_test_pass_five_${SECRET_TEST_PASS_FIVE_VERSION}
+  test_pass_six:
+    external: true
+    name: ${STACK_NAME}_test_pass_six_${SECRET_TEST_PASS_SIX_VERSION}