Spells that don't need attack rolls.
Are there any abilities for a Spellcaster to attack with ranged spells in melee distance, without imposing disadvantage?
Yes, there are such abilities; and every spellcaster already has them. All spellcasters have spells that can be used offensively at range but which don't require a ranged spell attack roll, and as such attract no penalty when used while the caster is within 5' of a hostile creature.
A select few spells don't involve any rolls at all; magic missile is the most commonly-used example, but power word kill or stun also fall into this category.
More common are spells that call for saving throws from the target or targets, instead of making a spell attack. At most levels, for most spellcasting classes, there are saving-throw based attacks that are comparable to the attack-roll based spells of the same levels. Often the saving-throw based spell attacks multiple targets in a (small) area, while the attack-roll based spell affects only one target (but there are exceptions to both of these).
So a spellcaster that expects to be within 5' of hostile creatures some of the time might want to invest in non-attack roll spells — acid splash or poison spray instead of firebolt; shatter instead of scorching rays, and so on.
In fact, most spellcasters only have a small number of spells that call for ranged attack rolls -- according to the SRD (searching via D&D Beyond):
Cleric: guiding bolt.
Druid: produce flame.
Sorcerer: chill touch, firebolt, ray of frost, scorching ray.
Warlock: chill touch, eldritch blast, ray of enfeeblement.
Wizard: chill touch, firebolt, ray of frost, acid arrow, ray of enfeeblement, scorching ray.
If you are using the SRD, it is only those nine spells (out of more than 300) that are penalized when cast within 5' of a hostile creature. The PHB and other sources add more, of course, but they also add many additional spells that don't require attack rolls.